The present invention relates to power control of transmitter equipment in a communication system, and more particularly relates to accurately controlling changes in transmission power levels.
Cellular communication systems are well-known and are in wide-spread use around the world. FIG. 1 is a diagram illustrating a common feature found in most systems: a serving node 101 (depending on the system, it can be called a “base station”, a Node B, an evolved Node B (“eNodeB” or “eNB”)) serves user equipment (UE) 103 (e.g., a mobile terminal) that is located within the serving node's geographical area of service, called a “cell” 105. For convenience, the term “serving node” will be used henceforth throughout this document, but any such references are not intended to limit the scope of the invention to any one particular system. Thus, references to “serving node” are intended to also refer to “base stations”, “Node B's”, “eNodeB's”, “eNB's”, and also to any equivalent node in a cellular communication system.
Communication is typically bidirectional between the serving node 101 and the UE 103. Communications from the serving node 101 to the UE 103 are referred to as taking place in a “downlink” direction, whereas communications from the UE 103 to the serving node 101 are referred to as taking place in an “uplink” direction.
In cellular communication systems, such as but not limited to the Global System for Mobile communication (GSM), Code Division Multiple Access (CDMA), Time Division-synchronous CDMA (TD-SCDMA), Wideband CDMA (WCDMA) and Long Term Evolution (LTE) systems, it is required to adapt the output power level of a transmitted signal based on the signal strength at the antenna port of the transmitter which, for example, can constitute part of UE or mobile station (MS) or serving node. Depending on which transmitter it is whose power is to be controlled (i.e., the serving station's transmitter or the UE's transmitter), dedicated commands for controlling transmitter power are deployed from the MS/UE or from the serving node of the different cellular communication systems.
In cellular communication applications, the accuracy of the absolute transmit power level is sometimes less important than the accuracy of a relative level change of the transmission power. An example of this can be found in the Third Generation Partnership Project (3GPP) WCDMA standard for mobile user equipment. It will be understood that references made herein to the 3GPP WCDMA standard are made for purposes of illustration only, and are not intended to express or imply that the various concepts and principles discussed herein are limited to systems that operate in accordance with that standard. To the contrary, the various inventive aspects described below are usable in other systems as well.
Transmission power level test patterns defined by the 3GPP specification are depicted in FIG. 2. Different test intervals A, B, C, D, E, F, G, H are shown. In that system, the relative step accuracy between power levels is an extremely tight requirement, especially during test intervals E and F, which call for descending 1 dB steps (test interval E) followed by ascending 1 dB steps (test interval F). For each 1 dB ascending or descending step in power, the relative accuracy is required to be +/−0.5 dB. Across conditions, transmitter path gain adjustments, and the entire 74 dB dynamic range, this can be extremely challenging without time-consuming factory calibration.
Component spreads as well as performance variations due to temperature and supply fluctuations as various applications are run require a dedicated gain calibration for nearly each gain (or power) step to meet the 3GPP specification requirements. This is especially so for those points in the dynamic range where large gain reconfigurations are made to optimize current consumption across output power. Often the error originates from imperfections in the analog paths of a UE or serving node transmitter.
Two different architectures are commonly deployed in mobile cellular communication systems to cope with this problem:                Software (SW) supported feed-forward methods (Open-loop)        Hardware (HW) based correction loops (Closed-loop)The inventors of the present invention have recognized that these commonly used approaches each have disadvantages, as will now be discussed.        
Looking first at SW supported feed-forward (Open-loop) methods, these require a relatively high calibration effort to characterize a particular UE and establish a custom correction table for that unit's gain across output power. This form of unique part-to-part product calibration can become expensive. The achieved accuracy is moderate, and performance margins degrade when the deployed components are temperature sensitive, even to the point of failure depending on the amount of calibration performed. As the number of power steps for which compensation is required increases, memory size and calibration time increase significantly, and depending on the drift and temperature stability/predictability of the components, parts can fail to satisfy specification requirements. Most 3G WCDMA solutions in the market presently, however, take advantage of a more limited conformance requirement to the specification, employ this technique and pay the burden of higher calibration costs in order to benefit from a less complex hardware solution without any feedback path, and manage to save on hardware components for detection and implementation of the loop.
In contrast to the SW supported feed-forward (Open-loop) methods, the HW supported correction techniques (Closed-loop) require additional components to capture/detect an appropriate representation of the transmitted signal and then process it for adjustment to the transmitter path gain. As a result of the additional signal manipulation, the required total component count can increase (depending on implementation). Despite this drawback, the achievable system performance can improve significantly due to the error compensation. The comparison of target absolute and/or relative signal levels to real measured signal levels results in an error signal that, through compensation, can be reduced almost to zero. Unfortunately, drift, imperfections due to component spread, load variations and supply voltage dependencies in the feedback path can degrade the overall accuracy of HW supported correction loops. Partial compensation for these imperfections can be achieved via smart calibration techniques in production, but the afore-mentioned goal of Closed-loop approaches is, of course, to minimize calibration as much as possible, and the implementation needs to be as robust as possible in this regard.
It is a common limitation of all HW correction loops that the available detector technologies have a smaller dynamic range than is required to operate over the entire dynamic range specified by the 3GPP WCDMA standard, and so all HW correction loops need to operate as SW supported (Open-loop) methods at lower powers. Differences in detector technology and associated detector dynamic range result in different systems having different ranges of closed-loop operation, but all of them, at some point, fall-back to Open-loop operation at lower powers. The importance of the Closed-loop correction is typically in the higher and mid power ranges to address the limitation of maximum power across conditions, maximum power compression of the transmitter path/power amplifier, gain expansion of the power amplifier in the mid-power range, and any large gain re-configurations to optimize direct current (DC) consumption of the power amplifier around mid-power levels.
Conventional HW supported correction loops come in several forms and have different advantages and disadvantages. One form utilizes a purely analog feedback path; another involves some digital processing. The purely analog feedback loop is most-often based on coupling off a representation of the transmitted signal, detecting it as an absolute output power level, comparing the detected absolute power level to the desired absolute power level (of the complex baseband signal, for example), and making corrections to increase or decrease the output power using analog adjustment of a real-time dynamic closed-loop until the difference between the wanted and detected absolute power levels is minimized. This analog loop can pose challenges with respect to stability; time delay adjustment; coupling losses; detector nonlinearity; and absolute power accuracy as a function of frequency, environmental conditions, detector errors, and load mismatch at the output of the transmitter path. A disadvantage of this approach is that if the power is reduced by conditions that make it impossible to reach the absolute power target (for example, as a result of large output load mismatch on the power amplifier), this loop will overdrive into heavy compression and unintentionally destroy the transmitter's signal linearity. Another disadvantage is that the instantaneous envelope distortion in the feedback signal due to limited detector dynamic range can result in large errors. Consequently, the detector dynamic range is typically forced to be larger. The detector accuracy and linearity are also required to be excellent in order to correctly compare to the target reference (i.e., “wanted”) power level, and these factors all result in more expensive and power hungry detector technologies.
A second method, which also relies on the measurement and correction of “absolute” power levels, is one in which the transmitted signal's output power is coupled off, averaged heavily and filtered to reduce changes in envelope magnitude to arrive at an average root mean square (RMS) power. The power of the resultant signal is then detected and processed by digital techniques and compared to an expected (“wanted” or “target”) average output power (for example, from the complex baseband signal).
The filtering is required because often the peak-to-average power ratio (PAR) of the radiofrequency (RF) signal may vary significantly as a function of time depending on the deployed standards, the chosen coding rates and the selected modulation schemes. For instance, to achieve an RMS signal gain/power accuracy of better than 0.1 dB usually requires up to 50 μs of averaging (measurement) time. A timing delay of, for instance, 50 μs between measurement start and completion of the error correction procedure corresponds commonly to approximately 10% of the total available slot time for TD-SCDMA and WCDMA systems. Unfortunately, the later the correction is performed outside the brief +/−25 μsec window of the inter-slot intended for gain adjustment, the larger the residual error with respect to the wanted RMS burst power and the higher the impact to error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) will be. In addition, this technique is usually based heavily on software algorithms that further burden the delay of the correction, as opposed to faster methods. This technique still suffers (although less than the purely analog approach discussed above) from the averaged distortion due to detector limited dynamic range, and typically requires high-performance, expensive and power-hungry detector technologies. This technique does, however, benefit from the use of digital techniques to intelligently manage the amount of correction employed under any given condition.
The inventors have recognized that both of the above-mentioned Closed-loop approaches suffer from a large disadvantage in the presence of impedance variations at the antenna. The coupling and detection of the “absolute” transmitter output power relies heavily on isolation from any reflected power coming back from the antenna as a result of impedance mismatch. This reflected power has a significant effect by “leaking” into the desired detector path and corrupting the measurement of the absolute forward power, requiring extremely high directivity couplers, and often the addition of isolators in the transmitter path to further address this problem. These high-performance additional components add significant cost and size to each independent 3G band of the overall solution.